DESCRIPTION: (Adapted from the applicant's abstract and Specific Aims.) T-cell activation in response to antigen depends upon stimulation of the T cell antigen receptor and upon signals delivered through accessory T cell molecules, such as CD28. CD28 binds B7 (CD80) and B7-2 (B70), cell-surface molecules expressed by antigen-presenting cells. The current application seeks to define the structural basis for CD28- mediated signaling, with the goal of shedding light on the relationships between signaling pathways and cellular responses. Previous studies have shown that perturbation of CD28 induces its tyrosine phosphorylation. The first specific aim is to characterize the CD28- mediated signals that require cytoplasmic tyrosine residues. CD28 mutants with Phe for Tyr substitutions will be used to determine which Tyr residues are required in order for CD28 to elicit particular early signaling events and to mediate certain cellular responses. In addition, proteins whose interaction with CD28 requires these Tyr residues will be identified. Also, a 50 kD protein that binds CD28 at Tyr 170 and is closely related to phosphatidylinositol 3'-kinase p85 will be characterized. The second specific aim is to determine the importance of the interaction of CD28 with p72itk/emt which is a member of the tec family protein tyrosine kinases. The third specific aim will use a yeast two hybrid system to identify proteins that can interact with the cytoplasmic domain of CD28 independently of its phosphorylation.